This invention relates to mitigating vibro-acoustic energy and, more particularly, mitigating such energy in a launch vehicle payload fairing to reduce the resulting load on the payload and thereby decrease the probability of damage to the payload.
Vibration and acoustic energy generated by rocket engines are transmitted through the launch vehicle fairing and create severe vibro-acoustic loads that can damage payloads. The magnitude of vibration transmitted into the payload volume largely depends on the external noise levels, the mass, stiffness, and damping characteristics of the fairing, and the acoustic damping within the fairing volume. Composite fairings currently being developed are much lighter and stiffer than their metal-alloy predecessors. However, composite fairings typically have little inherent damping. The lack of damping results in structural resonances, which are very efficient paths for noise transmission, and are thus deleterious to a fairing payload.
Acoustic blankets are often attached to the inside of fairings to absorb acoustic energy, thereby reducing the overall response in the fairing. Acoustic blankets are typically fabricated using low-density fiber material or foam. Current acoustic blankets for launch vehicles are usually less than six inches thick because there are strict constraints on their mass and volume. As a result, such blankets do not provide much sound absorption below 250 Hz. Although acoustic blankets can damp acoustic resonances in the fairing, they do little to alter the structural dynamics of the fairing.
The physics of sound transmission through walls and panels, and the effect of sound attenuating blankets, have been well documented in the relevant technical literature. Fairing noise transmission can be considered as the coupling of two dynamic systems, the fairing structure and the acoustic volume enclosed by the structure, which is excited by an external distributed broadband acoustic field. The responses of both dynamic systems exhibit resonant behavior, which is determined by physical properties such as air density, structural stiffness, structural mass, structural damping, acoustic damping, the speed of sound, and geometry. A mathematical model of fluid-structure coupling and a further discussion of the physics is reviewed in L. D. Pope, “On the Transmission of Sound through Finite Closed Shells: Statistical Energy Analysis, Modal Coupling, and Nonresonant Transmission”, J. of the Acoustical Society of America, Vol. 50, No. 3 (Part 2), pp. 1004-1018 (1971); and in P. Gardonio, N. S. Ferguson and F. J. Fahy, “A Modal Expansion Analysis of Noise Transmission Through Circular Cylindrical Shell Structures with Blocking Masses” J. of Sound and Vibration, Vol. 244, Number 2, pp. 259-297 (Academic Press, 2001).
It has been observed that the vibration loads transmitted to the payload by the interior acoustic response are comparable in level to the vibrations transmitted by mechanical truss mounts. There has been much work to reduce the mechanical path transmission using isolation systems, but little progress has been made to reduce the acoustic path transmission. For the Cassini spacecraft launched using a Titan-IV booster, special acoustic blankets were designed, tested, and implemented to meet acoustic load requirements. This work is documented in W. O. Hughes, A. M. McNelis, and H. Himelblau, “Investigation of Acoustic Fields for the Cassini Spacecraft: Reverberant Versus Launch Environments”, AIAA-99-1985, (American Institute of Aeronautics and Astronautics, 1999). As noted by Hughes et al., there has been little development in aerospace acoustic blankets since the 1970's. They also found that acoustic blankets are typically effective only above 400 Hz.
Acoustic blankets provide absorption (or damping) of pressure waves, which means that acoustic energy is converted to heat by interacting with the blankets. The amount of interaction between the blanket and the sound field is frequency dependent. In order to damp low frequency sound, thicker blankets are required. The amount of damping added to an acoustic resonance is determined by impedance matching between the material and fluid waves, material thickness, surface area coverage, and the ability of the material to dissipate energy, with the latter being ultimately limited by mass and volume constraints.
Fairing noise control for launch vehicles is similar to general aircraft noise control, which has been thoroughly researched. In one solution for aircraft, vibration absorbers are attached to the fuselage skin to reduce the structural vibration and sound radiation by the aircraft's skin. C. R. Fuller and J. P. Maillard, “Control of Aircraft Interior Noise Using Globally Detuned Vibration Absorbers”, J. of Sound and Vibration, Vol. 203, No. 5, pp. 745-761 (Academic Press Limited, 1997). As noted therein, interior noise response is reduced in two ways: first, the devices can couple to structural resonances and add structural damping, thereby reducing the amplitude of the resonant response (classical tuned mass damper approach); and secondly, by interacting with and breaking up the response of structural modes that radiate acoustic energy, often referred to as “modal reconfiguration”. A similar approach was discussed in the previously referenced paper by Gardonio et al., supra, wherein it is shown that discrete masses added to the fairing shell could reduce coupling between structural modes and the acoustic modes. It has also been shown that passive dynamic absorbers can couple to the fairing structure and significantly reduce interior noise transmission. S. Griffin, S. Lane, C. Hansen, and B. Cazzolato, “Active Structural-Acoustic Control of a Rocket Fairing Using Proof-Mass Actuators,” Journal of Spacecraft and Rockets, Vol. 38, No. 2, pp. 219-225 (American Institute of Aeronautics and Astronautics, Inc., March-April 2001).
There are a number of devices that enhance noise reduction by increasing sound absorption through a panel. For example, U.S. Pat. No. 4,384,634, titled “Sound Absorbing Structures,” uses a perforated face sheet attached to a honeycomb panel to provide acoustic damping, and a viscous elastic layer between the panel and vibrating structure, e.g., a jet engine, to damp structural motion. This treatment, however, is generally too heavy for a fairing and would not provide as much broadband acoustic absorption as a conventional foam blanket. Furthermore, viscous elastic treatments are not effective in coupling with low frequency structural resonances and in providing damping.
U.S. Pat. No. 4,560,028, titled “Sound Absorbing Wall Lining,” discloses a fluid-filled honeycomb panel where wave motion in the fluid provides acoustic damping. This invention is intended for waves in fluid mediums, such as underwater. Such a fluid-filled structure would weigh too much for use in launch vehicle fairings.
U.S. Pat. No. 4,667,768, titled “Sound Absorbing Panel,” shows a variation of the honeycomb panel. This patent teaches the reactive versus resistive resonator approach, and incorporates panel drains to release liquid build-up, and spheres to fill the honeycomb cell volumes in order to create varying acoustic resonances. This apparatus was designed for jet engine exhaust nacelles, and is not directed to increasing the transmission loss through a structure into an enclosed acoustic volume.
U.S. Pat. No. 4,842,097, titled “Sound Absorbing Structure,” also teaches reactive and resistive acoustic resonators bored into sound insulation material. Again, only acoustic damping, without providing any structural damping, is addressed. While the patent claims “substantial sound absorption at frequencies of less than about 1000 Hz”, it is a fact that as the target frequencies become lower, the volume of the resonators' cavities must become larger to reduce the effective air spring. Enlarging the volume of the resonators' cavities in accordance with the teachings of this patent would require geometries impractical for fairing applications intended to attenuate acoustic frequencies below 200 Hz.
U.S. Pat. No. 5,024,290, titled “Sound Absorbing Panel for Interior Walls,” uses grooves, funnels and holes in an attempt to augment the acoustic damping provided by the panel. The holes create a reactive and resistive acoustic resonator effect. What is not discussed in these reactive and resistive resonator patents is that as the resonator damping (the resistive part) increases, the coupling of the device to the acoustic volume decreases. Also, such resonators are essentially wasted volume at high frequency because they provide no acoustic absorption. Thus, it is often preferable in practice to maximize the amount of acoustic foam in order to optimize high frequency attenuation instead of implementing resonators.
In U.S. Pat. No. 5,824,973, titled “Method of Making Sound Absorbing Laminates and Laminates having Maximized Sound Absorbing Characteristics,” provides an informative discussion on the role of acoustic impedance and impedance mismatching in designing optimal blanket treatments. In addition to discussing the physics and dynamics of multiplayer porous acoustic absorption laminates, it provides plots of normal incidence sound absorption for a variety of configurations. In each case, it is observed that blanket performance is nearly non-existent at frequencies below 200 Hz. Thus, no benefit would be observed in a structural-acoustic system where resonances occur below 200 Hz. As is the case for nearly all acoustic blanket approaches, no consideration has been given to the structural input path.
U.S. Pat. No. 5,910,082, titled “Sound Absorbing Building Panel,” teaches a panel for use as a wall or ceiling tile. The innovation lies in the use of “sound absorbing” granules, which are adhered to one another to form matrix. Such an approach may be beneficial for high frequencies, but it will not damp low frequency acoustic resonances. At low frequency, the sound waves will see the matrix layer as a single, lumped panel and propagate across the panel without attenuation. This treatment is thus ineffective for low frequency absorption or for mitigating structural-acoustic transmission in lightly damped structures.
U.S. Pat. No. 6,090,478, titled “Sound Absorbing/Shielding and Electric Wave Absorbing Plastic Sheet Containing Encapsulated Magnetic Fluid, and Sound Absorbing/Shielding and Electric Wave Absorbing Plastic Panel,” discloses a lightweight sound absorbing material with insulating effects. The invention comprises a “sound absorbing and insulating” plastic panel, with glass “balloons” or magnetic fluid-filled capsules. The fluid-filled capsules provide electromagnetic shielding at high frequencies (800 MHz). The use of microcapsules of fluid dispersed randomly throughout a plastic composite, however, cannot significantly mitigate low frequency structural-acoustic transmission outside of mass loading benefits, which could be better achieved with a softer and higher density material. Moreover, even of one were to overlook the aforementioned shortcomings, the density of such a panel would result in a weight that would far exceed allowable weight limits for launch vehicles. In launch vehicles, the acoustic treatment must be lightweight and compact.
U.S. Pat. No. 6,109,388, titled “Sound Absorbing Mechanisms Using a Porous Material,” presents another panel that incorporates imbedded acoustic resonators. As with other efforts for sound absorbing panels, this invention focuses only on absorbing or damping the acoustic space without addressing structural transmission.
There are some devices that seek noise reduction by preventing or reducing the transmission of noise, as opposed to enhancing acoustic absorption. For example, U.S. Pat. No. 5,504,282, titled “Sound Transmission and Absorption Control Media,” teaches a sound transmission barrier for boats and motor vehicles using multiple “high-mass” layers. In launch vehicles, it is desirable to couple to and damp low frequency structural modes with minimal added mass.
U.S. Pat. No. 5,907,932, titled “Wall Structure Having Enhanced Sound Transmission Loss,” discloses two walls separated by an isolating barrier. To extend the transmission loss to low frequencies, however, the walls must be de-coupled, which requires that the combined structure be relatively thick. This is often impractical, as in the case of launch vehicle fairings.
There are also many apparatus that address both sound and vibration transmission. Typically, the panel absorption is enhanced if the panel is fabricated from a non-homogeneous material, and vibration damping is provided through molecular friction resulting from wave propagation through a viscous elastic layer. U.S. Pat. No. 5,400,296, titled “Acoustic Attenuation and Vibration Damping Materials,” issued to Cushman and Thomas, discloses a material or barrier intended to provide both acoustic absorption and vibration damping. Particles such as glass spheres, steel pellets, brass pellets, or chunks of lead or cork, are arbitrarily or randomly dispersed in a matrix of some type of urethane, silicon rubber, elastomer, polymer, gypsum, or petroleum extract. There is no tailoring or designing of the vibration damping mechanism to the resonance frequencies of a structural application, e.g., an aircraft fuselage or launch vehicle fairing. The only vibration attenuation mechanism taught by the foregoing device is the damping of waves as they propagate through the material, which results from internal molecular friction. This type of approach yields weak coupling and provides little dissipation of the structural resonances of the host structure.
The key innovation of the '296 patent is the use of very small (≈100-micron diameter) particles in the elastomeric matrix to interact with wave energy propagating in the material in order to convert the wave energy from one type of wave, e.g., bending, to another type of wave e.g., torsional. It is asserted that mechanical energy is converted to heat through the wave transformations that are expected to occur, thereby reducing the transmission of energy through the material. Propagating energy is diffused. The efficiency of this mechanism is a function of the wavelength of the energy being propagated, the wave speed of the matrix material, the directionality of the incident acoustic energy, and the material density. At low frequency, sound waves would pass through the material with little to no effect unless the thickness of the material was on the order of ¼-wavelength of the incident sound wave.
U.S. Pat. No. 5,526,324, titled “Acoustic Absorption and Damping Material with Piezoelectric Energy Dissipation,” issued to Cushman, discloses piezoelectric particles embedded within a matrix material. It is asserted that the particles act as electrical short circuits and are able to convert acoustic and vibration energy into heat through electrical discharge. This approach would be significantly heavier that traditional blankets since it requires electrically conductive or active materials and piezoelectric particles. Its impedance matching to the acoustic space and broadband attenuation would be inferior to that provided by foam. The particles are said to be randomly distributed. This neglects transmission loss at low frequency. U.S. Pat. No. 5,706,249, titled “Panel Spacer with Acoustic and Vibration Damping,” also issued to Cushman, teaches an acoustic and vibration dampening spacer to hold apart and transmit loads between multi-panel walls. Mechanical waves propagating in the panel, whether resulting from acoustic loads or structural vibration, are dissipated by an elastomeric material. The disclosed device is simply an elastomer spacer placed between panels, and is not related to a sound absorbing or sound barrier treatment suitable for fairings.
U.S. Pat. No. 5,745,434, titled “Acoustic Absorption and Damping Material with Integral Viscous Damping,” also issued to Cushman, is basically the same concept originally presented in U.S. Pat. No. 5,400,296, but employs a material composed of discarded tire rubber and has channels and passage ways through a matrix to allow for more effective acoustic coupling. Obviously, such a product would be very dense, have poor acoustic coupling ability, and would reduce structural transmission primarily by increasing the mass and thus the impedance load. It would also be subject to the same bandwidth limitations that were noted in the '296 patent. U.S. Pat. No. 5,754,491, titled “Multi-technology Acoustic Energy Barrier and Absorber,” yet another patent issued to Cushman, also uses the concept disclosed in the '296 patent, with the added innovation of using multiple layers, with each layer being separated by a viscous elastic layer intended to act as a constrained-layer damper.
U.S. Pat. No. 6,237,302, titled “Low Sound Speed Damping Materials and Methods of Use,” teaches a vibration and acoustic treatment that utilizes granular fill. In this invention, a granular material, e.g., lead shot, sand, or rice, is used to damp structural vibrations. The granular material is placed in “intimate contact” with the structural member. The invention asserts that structurally radiated noise can be reduced by the granular damping treatment. However, the patent describes only means to damp structural vibration; it provides no means to absorb acoustic energy. The process taught therein would significantly increase the mass of the structure. Furthermore, the foregoing approach is not designed to target those frequencies that most contribute to noise transmission.
As previously noted, the fairing noise problem is similar to the aircraft cabin noise problem. U.S. Pat. No. 5,904,318, titled “Passive Reduction of Aircraft Fuselage Noise,” presents the concept of using a reinforced skin structure to mitigate interior noise. A combination of air barriers, insulating layers, and dampening rods is incorporated into double or multiple layer panels of a fuselage, which is impractical for a fairing. The effect is to decouple the various panel layers and increase the damping of structural resonances. This patent is noteworthy because it provides a good explanation of the structural-acoustic coupling between the external loads, fuselage skin, and the enclosed acoustic space. It is pointed out that “vibrating skin panels can often act as an efficient loud speaker, radiating noise into the interior panels and into the cabin of the aircraft.” The patent discusses the use of acoustic blankets and their limitations. It also discusses the use of viscous elastic treatments that are applied directly to the vibrating fuselage in an attempt to add damping to structural resonances. At low frequency, however, there is little strain energy transferred to the viscous elastic material, and thus little damping is added to the low frequency dynamic response.
Several references directly address the vibro-acoustic response in launch vehicle fairings. U.S. Pat. No. 5,670,758, titled “Acoustic Protection on Payload Fairings of Expendable Launch Vehicles,” and U.S. Pat. No. 6,231,710, titled “Method of Making Composite Chambercore Sandwich-type Structure with Inherent Acoustic Attenuation,” both teach the use of Helmholtz resonators imbedded in the fairing wall to couple to and attenuate low frequency acoustic resonances in the fairing volume. However, no test data is presented in either patent that demonstrates that this can actually be achieved. In the '758 patent, horn-shaped or cup-shaped resonators are inserted into tile panels of acoustic foam and adhered to the fairing wall. In the '710 patent, double panel walls of the fairing are used as the resonator volumes, with orifices cut into the fairing structure to permit coupling to the acoustic field. Neither invention addresses the structural transmission path, nor is either designed to reduce the amplitude of structural resonances.
U.S. Pat. No. 6,195,442, titled “Passive Vibroacoustic Attenuator for Structural Acoustic Control,” teaches the use of a combined structural and acoustic approach. However, this device is designed specifically for low frequency acoustic modes and is intended as an add-on to existing foam treatments. The acoustic mitigation is provided by a tuned diaphragm, membrane structure, or even acoustic resonator, and is not intended to provide the broadband acoustic dissipation of an acoustic blanket.
There is need in the art for a lightweight acoustic barrier between a structure and an enclosed acoustic volume that can optimally interact and dissipate structural resonances of the enclosing structure that contribute to noise transmission into the enclosed acoustic volume, while simultaneously affording maximum dissipation of the broadband interior acoustic response. The present invention addresses this need in the art.